1. Field of the Invention
The invention relates in general to a projection apparatus, and more particularly to a projection apparatus, which reduces effects of image flare and stray-light leak by using different filtering parts or reflection areas to maintain image luminance.
2. Description of the Related Art
The projector provides an illumination light beam via an illumination system, transforms the illumination light beam into an image light beam via a digital micro-mirror device (DMD), and projects the image light beam from a projection lens onto a screen to form an image. An f/number of the projection lens is used to control the luminance of the image. The smaller the f/number is, the larger the radius of the stop and the amount of light passing through the stop is, and the larger the f/number is, the smaller the radius of the stop and the amount of light passing through the stop is. Therefore, when one designing a projector, the projection lens with a smaller f/number is usually used to improve the luminance of the projector.
Referring to FIG. 1, a ray-fan diagram of a projection lens using a smaller f/number (i.e. f/2) is shown. From FIG. 1, it can be known that red, green and blue light with different wavelength has different aberration on an image-formation plane. Especially, the large-angled blue light with shorter wavelength is obviously deviated from a normal image-formation position, which results in a serious flare at the outer periphery of the blue light and thus image flare make the clear focusing become difficult. In the conventional projection lens or illumination system, an opaque block plate is added to block the large-angled light from entering the projector. However, this method also blocks the large-angled red and green light in addition to the large-angled blue light, which reduces luminance of the projector.
Besides, a dark-coating material is coated in the region outside of an effective area (i.e. an area with a micro-mirror array) of the DMD to reduce stray light. The dark-coating material has different reflection rates for the light with different wavelength and usually has a higher reflection rate for the blue light with shorter wavelength. The reflected stray light generates blue light leak through the projection lens, which provides extra blue color at parts of the projection image. Especially, when the image is a dark frame, i.e. every micro-mirror of the DMD is in an off state, the effect of local blue-light leak is observed more clearly. The conventional solution is to add an opaque block plate in the illumination system to block the off-state light from entering the projection lens. However, this method similarly blocks the red and green light in addition to the blue light, which reduces the luminance of the projector.